This invention relates to an improvement in cutting devices for felling trees, and more particularly to such devices comprising two power operated knife blades of curved shape mounted to be swung towards and away from each other.
U.S. Pat. No. 3,540,501 discloses a device of this kind, where two knife blades are mounted on swingable arms journalled in bearings on the same shaft. Each of the blades have a cutting edge and is curved in a cross section perpendicular to the shaft to form an arc having a radius substantially corresponding to the distance from the center line of the shaft. The principal direction of each of the two cutting edges form an angle of at most 70.degree. with the shaft. The knife blades are also curved in parallel to the edges, thus together forming a bowl, preferably a segment of a sphere.
The fact that the knife blades may be curved in two transverse directions means that they can be comparatively thin and still retain enough rigidity to transmit the considerable forces required to move the blades through a big tree trunk. Inherently, thin knife blades need lower forces to cut than do thicker knife blades, and they also inflict less damage by compression and splintering to the trunks severed.
Felling heads of this known kind have been good performers. The cutting blades have proved to withstand normal abuse for long periods in spite of cutting close to the ground and leaving very low stumps. However, in cases where sawlogs and plywood bolts are recovered from trees felled with such prior heads, the damage in the form of cracks in the butt end of the logs is not insignificant.
In the middle area of a tree, where the cutting edges move almost at a right angle to the grain, the wood is often not cut, but rather torn apart. This is because the blades, though being comparatively thin, act like wedges in their travel towards each other. It happens that this wedge action causes cylindrical annual ring sections of considerable lengths to be pulled out.
When felling large diameter trees with a device according to U.S. Pat. No. 3,540,501, the edges of the opposed curved blades will cut through the outer layers of the trunk at an acute angle to the grain. It has been observed that during this initial phase of the cutting operation, very smooth surfaces are generated and that the power required to move the blades is noticeably low. Splintering action is insignificant.
It is the object of the present invention to improve the known felling heads with opposed curved cutting blades to enable such heads to retain these good features from the initial phase all through the cutting operation.